The present invention relates generally to methods for making printed circuit boards and more particularly to a method for fusing solder to said boards.
Conventional printed circuit boards typically comprise a copper clad plastic substrate between about 0.020 and 0.060 inches in thickness, into which a plurality of holes are punched or drilled that are used either to mount various components or connecting wires. To produce the circuit, a photoresist is applied to the surface and is developed and holes are made. A coating of copper is then electrolessly plated onto the developed substrate. Then a layer of copper is electroplated onto the layer of electroless copper. A mixture of tin and lead is then plated on top of the copper. The photoresist is then removed and the copper surface is etched to produce the desired circuit pattern. Finally a tin lead solder layer is fused to the etched copper. The board as prepared is then ready for installation of electronic components.
The tin lead solder layer is typically comprised of about 58-70 weight percent tin, with the balance comprising lead, and it is deposited to a thickness of between about 300 and 650 microinches onto the copper surface. When initially deposited, this mixed metal layer basically comprises an mixture of lead and tin globules rather than a true alloy or a solid solution.
While the affinity of these metals is such that a certain amount of alloy formation occurs at room temperature, current practice is to speed up this process by heating the plated board to a temperature above the melting point of solder at which time the tin and lead dissolves therein and a solder alloy is formed. At the present time, the speedup of alloy formation is usually accomplished either by immersing the board in a bath of a hot oil held at a temperature just above the melting point of the solder alloy or by heating the board with an infrared lamp. In both of these methods it has been found that the thermal inertia inherent in these systems tends to retard the quick cooling the molten solder alloy so that a certain amount of fluid flow occurs. This flow usually results in a buildup of solder along the sides of the plated hole and a depletion of the solder around the rims of the holes. A certain level of rim solder is necessary to promote proper solder joint formation. However, if this level is not present, this rim depletion or "weak knee" condition results in a situation where there may not be enough solder to prevent the exposure and subsequent oxidation of at least a part of the underlying intermetallic layer. When this happens a proper fillet cannot form and an unacceptable fillet results.
For example, the hot oil technique typically uses a preheat solder bath at temperature of about 200.degree. F. and a second higher temperature solder at about 400.degree. F. to perform the fusing. However, in practice, it has been found that a temperature gradient often exists across the higher temperature solder bath which is as much as 10.degree.-15.degree. F. This differential heating across the bath results in differential heating of the boards which causes poor fusing. Furthermore, heating of the boards to a temperature of about 400.degree. F. results in excessively liquifying the solder so that the solder tends to pull away from the rim of the holes in the boards. This results in the aforementioned "weak knee" condition. It also exposes the underlying intermetallic layer to oxidation. This oxidation causes poor solder joints when components are soldered to the board. An article entitled "The Fusing of Tin-Lead Plating on High Quality Circuit Boards", by B. D. Dunn, published in Transaction of the Institute of Metal Finishing, Vol. 58. pp. 26-28 (1980) discusses the hot oil technique. In the infrared heating method, the variation in temperature across the board is not controllable at all, and this also results in very poor fusing of the tin-lead mixture.
Accordingly, it is an objective of the present invention to provide for a method of fusing tin-lead coated printed circuit boards that prevents the formation of weak knee conditions at the edges of holes in the boards.